The present invention relates to a method for the deposition of a thin film onto a substrate by the technique of chemical vapor deposition and in particular to a deposition process wherein the conditions during a first nucleation stage differ from the conditions during a second deposition stage for the bulk deposition of the film by means of chemical vapour deposition.
Such a process is known from WO 99/28527 from Applied Materials Inc. This document describes the chemical vapour deposition of a tungsten layer, using SiH4 and WF6, comprising a first deposition stage to facilitate nucleation of the film, followed by a second deposition stage. The technique of chemical vapor deposition to deposit solid thin films on substrates using one or more gaseous reactants has been known for many years. After inserting the substrate into a reaction chamber and closing the reaction chamber the chamber is evacuated. The substrate is heated to a temperature required for the reaction to proceed at an economical rate and the gaseous reactant or reactants are introduced into the reaction chamber. During the process the conditions like temperature, reactant flows and pressure are typically maintained at a constant value. The reactants or reactive species adsorb on the surface of the substrate and decompose or react with each other under the formation of a solid film and the release of gaseous by-products. One of the problems encountered in the chemical vapor deposition technique is that many processes are afflicted with a nucleation process which depends on the condition of the underlying surface. This means that the tendency of the reactants to adsorb on the surface, decompose and form a solid film depends on the condition of the surface of the substrate. In some cases this can be advantageously applied, e.g. in selective processes where the deposition of a film over exposed silicon surfaces is desired but not over parts of the substrate surface which are covered with silicon oxide. In order to achieve this result, the conditions can be chosen such that the nucleation rate on silicon is orders of magnitude higher than the nucleation rate on silicon oxide. After the surface is covered with a thin film of the material, the further deposition typically proceeds at a faster rate. In many cases, however, this phenomena leads to undesired effects, e.g. when a uniform deposition over all parts of the surface is desired.
In the prior art the problems related to a surface dependent nucleation are overcome by introducing a first deposition stage in the chemical vapour deposition process with process conditions that are favourable for nucleation of the film to be deposited and which are different from the conditions in the second and main deposition stage where the remainder of the film is to be deposited. However, conditions favorable for rapid nucleation are not necessarily favorable for uniform nucleation and/or deposition over the entire wafer surface as well as in trenches in the wafer surface.
Furthermore, the nucleation rate on parts of the reaction chamber can be different from the nucleation rule on the substrate. As a consequence, the nucleation stage on some inner surfaces in the reactor can be completed earlier th on other surfaces. When the nucleation stage is completed, film deposition starts, which leads to a higher consumption of the reactants. This means that the local conditions in the reaction chamber change in the course of the time during the nucleation phase which influences the controllability of the process in a negative way. For example, a linear extrapolation of the film thickness with the deposition time will give an unreliable prediction of the actual film thickness achieved. In addition, in the case of a batch reactor, not all wafers face a same local environment. In many cases the substrates have a wafer-like shape i.e. are thin in one dimension and much larger in the two other dimensions and in a batch reactor they are placed in a row, with their large surfaces normal to the direction of the row and at some mutual distance. In this case most of the substrates face other substrates to be processed. However, the first and the last wafer face an other local environment which can result in a significantly different film thickness on the substrate at the end of the deposition process. Also the presence of test wafers within a batch of product wafers results in a locally different environment.
Another problem related to the nucleation stage of the process is surface roughness. When nuclei are formed, they grow out in both horizontal and vertical directions till the complete surface area is covered. This process results in a relief of the surface. When the density of nuclei is high the relief is small and when the density of nuclei is low, the relief is pronounced. This relief will typically remain when the growth proceeds.
It is the object of the present invention to provide in a method of in-situ pre-treatment prior to the execution of the chemical vapor deposition process that avoids these disadvantages and produces a homogeneous starting surface for all surface areas within the reaction chamber.
The present invention proposes to perform the nucleation in the first stage of the process by Atomic Layer Deposition. According to one aspect of the invention a method is provided for chemical vapor deposition of a film onto a substrate comprising:
inserting a substrate in a reaction chamber,
subjecting said substrate to a first treatment comprising a nucleation treatment at a first temperature,
followed by a second treatment comprising bulk chemical vapor deposition at a second temperature
wherein,
said nucleation treatment comprises atomic layer deposition, wherein the substrate is alternatingly and sequentially exposed to pulses of at least two manually reactive gaseous reactants, said first temperature being chosen to prevent condensation of either of said reactants and to prevent substantial thermal decomposition of each of said reactants individually.
In the method according to the invention, a substrate or a plurality of substrates is inserted into the reaction chamber, the reaction chamber is closed and evacuated and the substrate is heated to a first temperature. At this first temperature at least two reactants are alternately and sequentially introduced into the reaction chamber. This first temperature is high enough to prevent condensation of the reactants on the substrate surface but so low that no thermal decomposition of the individual reactants occurs. Consequently, during the supply of the first reactant only a monolayer of the reactant chemisorps on the surface and then the surface is saturated. When the second reactant is supplied it chemisorps on the surface, reacts with the previous reactant and forms a solid monolayer until the surface is fully saturated. This cycle can be repeated a number of times. This method is called Atomic Layer Deposition and is much less susceptible for surface influence than normal chemical vapor deposition. Each reaction is self-limiting. It is possible to carry out this method with more than 2 reactants, e.g. 3 reactants for the deposition of a ternary compound, provided that the reactants are matching and, when introduced into the reactor separately, do not thermally decompose at the first temperature. Due to the deposition of a thin film by Atomic Layer Deposition, a uniform starting surface is provided for the subsequent chemical vapor deposition on all surfaces in the reaction chamber. After the Atomic Layer Deposition pre-treatment, the substrate is heated to a second temperature and the reactants for the chemical vapor deposition process are introduced into the reactor. After completion of the deposition by chemical vapor deposition the supply of reactants is cut-off and after evacuating and/or purging the reaction chamber and backfilling it to atmospheric pressure, when required, the substrate is removed from the reaction chamber.
In a first embodiment the thin film deposited by the Atomic Layer Deposition pre-treatment has mainly the same composition as the film deposited by chemical vapor deposition. In a second embodiment the thin film deposited by the Atomic Layer Deposition pretreatment has a different composition than the film deposited in the chemical vapor deposition step. In the case of layers, deposited in the two deposition stages, with substantially the same composition, the first temperature at which the Atomic Layer Deposition is carried out is typically lower than the second temperature at which the chemical vapor deposition is carried out, but this depends also on the reactants chosen.
In Atomic Layer Deposition processes the deposition rate is very low because of the required purge times in between the respective reactant pulses. Therefore, the reactor volume and shape of an Atomic Layer Deposition reactor is optimized to yield minimum purge times and a maximum deposition rate. Inherently to the Atomic Layer Deposition process is at the process conditions like pressure, temperature and flow are not critical as long as sufficient reactant is supplied. Because the process is based on the sequential and alternating saturation of the surface with one monolayer of reactant, a uniform film deposition will be obtained whenever saturation of the surface is achieved in the repeated cycles. On the other hand, reactors for regular chemical vapour deposition are optimized for uniform film deposition at an economical deposition rate. In chemical vapour deposition, temperature, pressure and reactant flow are critical parameters and the design of the reactor takes this into account to achieve a uniform deposition. This reactor design is not optimum for economical Atomic Layer Deposition. However, when Atomic Layer Deposition is applied as an in-situ pre-conditioning prior to chemical vapour deposition in a reactor designed for regular chemical vapour deposition, a limited number of Atomic Layer Deposition cycles with a time duration that is not economical for Atomic Layer Deposition are allowable to yield an overall process that is still economical. One of the measures taken to reduce the cycle time of the Atomic Layer Deposition pulses is to maintain the pressure inside the reaction chamber at a low value to allow rapid purging of the reaction chamber volume in between the pulses. Preferably this pressure is maintained below 10 Torr and more preferably this value is maintained below 1 Torr.
The method of Atomic Layer Deposition results in a full coverage of the substrate surface and a layer by layer growth. Therefore the surface roughness which is a result of a nucleation stage is omitted and the surface after Atomic Layer Deposition is basically just as smooth as the starting surface.